The present invention relates to a solid-state image sensing device and an operation method thereof and, more particularly, to a solid-state image sensing device and an operation method for scanning a predetermined area of the solid-state image sensing device.
With the progress of manufacturing technique of a charge coupled device (CCD), the density of pixels in the CCD increases. Further, a number of image sensing apparatuses, such as a video camera and a digital still camera, which adopt scanning method for scanning a predetermined area out of a readable area of the CCD (partial scanning) to realize anti-vibration mechanism, electronic zooming mechanism, display of a reduced image on a liquid crystal display as a finder, and so on, have been commercialized. Furthermore, partial scanning is also applied to automatic focusing and automatic iris diaphragm control. Accordingly, various partial scanning method are used in dependence upon utilization purposes. Some digital still cameras have a function of partial scanning for taking sequence photographs in low resolution so as to increase the number of photographs taken in unit time, in addition to a function for taking a normal high resolution and high quality photograph.
An example of conventional partial scanning operation performed in a digital still camera will be explained below.
FIG. 10 is a view showing a configuration of an interline (interlace) scanning type CCD which is widely used at present. In FIG. 10, reference numeral 10 denotes photodiodes (PDs) which perform photoelectric conversion; 11, four-phase drive vertical charge coupled devices (VCCDs) for transferring signal charges of the PDs 10; 12, horizontal charge coupled device (HCCD) for transferring the signal charges transferred from the VCCDs 11 by line; and 13, an output amplifier for converting each signal charge representing a pixel, transferred via the HCCD 12, into a voltage signal.
The CCD as described above is operated by an operation unit 200 which controls VCCD operation signals xcfx86V1 to xcfx86V4 for operating the VCCDs 11 and HCCD operation signals xcfx86H1 and xcfx86H2 for operating the HCCD 12 and outputs as timing pulses at proper levels.
In this type of CCD, normally, signal charges stored in the PDs 10 are transferred to the adjoining VCCDs 11, then pairs of signal charges of adjoining pixels in the vertical direction are added by two adjoining lines. Thereafter, the signal charges added by two lines are sequentially read out via the HCCD 12 as signal charges of interlaced single lines. The combinations of two lines to be added are alternatively changed in each field period as shown by xe2x80x9cfield modexe2x80x9d in FIG. 10. Besides the aforesaid xe2x80x9cfield modexe2x80x9d scanning method, there is a method for reading each signal charge independently without adding the signal charges. In this case, charges stored in pixels either in the odd lines or in the even lines are alternatively scanned in each field period (referred to as xe2x80x9cframe modexe2x80x9d hereinafter).
Next, an output unit of the CCD is explained.
FIG. 11 is a schematic illustrating a configuration of an output unit of the CCD as an image sensing device. In FIG. 11, reference numeral 101 denotes a transfer gate of the last element of the two-phase drive HCCD 12 A signal charge is transferred to a floating diffusion gate 103 via an output gate 102 (normally, it is fixed to a predetermined potential) by operating the potential of the transfer gate 101 from High to Low. The signal charge transferred to the floating diffusion gate 103 is then converted to a voltage signal corresponding to the potential of the floating diffusion gate 103 by the output amplifier 13. The output amplifier 13 is usually configured as a source follower.
Further, reference numeral 104 denotes a reset gate which functions as a xe2x80x9cwallxe2x80x9d when storing signal charges by applying a Low potential and drains the signal charge in the floating diffusion gate 103 to a reset drain 105 by applying a High potential.
FIG. 12 shows conceptual views illustrating potential profiles of the elements shown in FIG. 11 and a flow of signal charges, and FIG. 13 is a timing chart of a pulse signal (xcfx86H2) to be applied to the two-phase drive HCCD 12, a pulse signal (xcfx86R) to be applied to the reset gate 104, and a resultant output voltage (Vccdout).
Referring to FIGS. 11 to 13, before reading a signal charge stored in each pixel (i.e., before time ts), residual charge in the floating diffusion gate 103 is drained at time tr. Below, the time tr is called xe2x80x9creset timexe2x80x9d, time tf is called xe2x80x9cfield through timexe2x80x9d, an output signal level at the field through time tf is called xe2x80x9cfield through levelxe2x80x9d, the time ts is called xe2x80x9cimage signal transfer timexe2x80x9d, and an output signal level at the image signal transfer time ts is called xe2x80x9cimage signal levelxe2x80x9d.
Regarding frequencies of operation signals for the CCD, the highest frequency is limited in dependence upon performance of the amplifier 13. An output from the CCD is applied with correlated double sampling (CDS) outside of the CCD, then amplified. In order to properly perform the above processes on the output from the CCD, sufficient duration of the output signal (Vccdout) at field through level as well as at image signal level is required.
Further, it is also necessary to remove effects of irregularity at the leading and trailing edges of operation signals for the HCCD 12 and the reset pulse xcfx86R. Further, there is a limitation due to frequency characteristics of the output amplifier 13. With the aforesaid limitations, the frequencies of operation signals for the CCD are usually about 10 MHz. Note, when it is assumed that there is no foregoing limitation, frequencies of operation signals capable of securing necessary transfer efficiency for obtaining a non-deteriorated image via the HCCD is higher than 10 MHz, and may be several times higher than that.
Next, an operation for scanning a central area of 640xc3x97480 pixels of a 960xc3x97600-pixel CCD is explained with reference to FIG. 14. In the 960xc3x97600-pixel CCD, the number of lines scanned in each field period is 300 either in the field mode or in the frame mode (in the field mode, signals of pair of adjoining pixels are combined to generate signals of 300 lines, and in the frame mode, signals either in the odd lines or the even lines of the 600 lines, namely 300 lines, are outputted).
First, right after transferring signal charges in the PDs 10 to VCCDs 11 at time t1 (in the field mode, right after the signal charges of two pixels are added in VCCDs 11 since charges are added as soon as they are transferred to VCCDs 11), VCCDs 11 are operated at high speed to transfer charges of first 30 (=(300xe2x88x92240)/2) lines to discard the charges to a drain via the HCCD 12 during period t2. In a video camera, this operation is performed during vertical blanking period.
Since it is necessary to keep normal transfer efficiency during the discarding of the charges of the 30 lines, the frequencies of operation signals at this time is set between about 300 kHz and about 400 kHz. Since capacitance of the VCCDs 11 is large, the frequencies of the operation signals for operating the CCD is limited to the above, at most.
The drain may be arranged in parallel to the HCCD 12, or downstream of the HCCD 12. Further, the drain may be substituted by a drain configured in the output amplifier 13. Further, according to FIG. 14, operation signals are also applied to the HCCD 12 during the period t2 for transferring from the VCCDs 11, the signal charges of the 30 lines to be discarded. However, there is a method for temporarily storing the charges to be discarded in HCCD 12. In this case, right after the charges of 30 lines to be discarded are transferred to HCCD 12, the charges are transferred via HCCD 12 at high speed (normally, at tens of MHz) to be discarded, during period t3.
After discarding the unnecessary charges of the 30 lines via the VCCDs 11 and the HCCD 12, signal charges of 240 lines are sequentially read out, by line, in accordance with a horizontal synchronizing signal in period t4. Accordingly, in a video camera, when the number of pixels of the CCD in each line is 960, it is operated at about 16 MHz. Out of information on each line of 960 pixels, information on the central 640 pixels is read out using a memory as a buffer, in accordance with an operation signal. After reading 240 lines, signal charges of the remaining 30 lines are discarded at high speed in period t5. The charge discarding operation, at this time, is performed in the same manner as that described above for discarding the charges of the first 30 lines, and the signal charges of the remaining 30 lines are transferred in accordance with operation pulses of 300 to 400 kHz during vertical blanking period so as to transfer the signal charges at normal transfer efficiency.
In the aforesaid partial scanning method, however, there are following problems.
(1) Signal charges of unnecessary lines are drained, however, in a line which includes necessary signal charges, signal charges of all the pixels in the line, including unnecessary signal charges, are also read out. Therefore, the number of pixels in a line must be within the number of pixels which can be read in a line display period at video rate. In present, operation speed in the horizontal direction is ten plus MHz, thus, the maximum number of the pixels in each line is limited to about 1000. Further, to scan all the pixels in the line and output a part of them in synchronization with the line display period at video rate, it is necessary to temporarily store image signals of all the pixels in memory and output image signals of the part of the pixels after changing time scale so as to match the line display period. This operation makes the control complicated.
(2) The limitation explained in the problem (1) is on a video camera. In a digital still camera or a camera for a personal computer, for instance, there is more freedom in operation speed. However, high operation speed for scanning a part of an image is still demanded. Accordingly, time for reading unnecessary charges in the conventional method prevents overall operation speed from increasing in the above devices.
(3) Furthermore, the vertical and horizontal blanking periods which are bounded by a conventional video rate are useless in a digital still camera which may be required to read an image at high speed. Moreover, in a digital still camera, there is a demand for shortening the time gap between when a shutter button is pressed and when an image is actually read out. In addition, scanning operation performed at a high frame rate is also required for photometry.
The present invention has been made in consideration of the above situation, and has as its object to provide partial scanning method capable of reading a predetermined area of a solid-state image sensing device at high speed and a solid-state image sensing device.
According to the present invention, the foregoing object is attained by providing a partial scanning method for reading a necessary charge from a predetermined first area of a solid-state image sensing device and discarding a unnecessary charge from a second area which is other than the first area, while scanning the solid-state image sensing device having pixel elements for storing charges, vertical charge transfer devices, arranged in parallel, for transferring charges stored in the pixel elements toward a horizontal charge transfer device, and the horizontal charge transfer device for transferring charges to output the charges to outside of the solid-state image sensing device, the method comprising: a first charge transfer step of transferring charges from the vertical charge transfer devices to the horizontal charge transfer device; a discarding step of discarding charges, out of the charges transferred in the first charge transfer step, obtained from the second area by operating the horizontal charge transfer device; a reading step of reading charges, out of the charges transferred in the first charge transfer step, obtained from the first area by operating the horizontal charge transfer device; and a second charge transfer step of, when there is a remaining charge in the horizontal charge transfer device after the reading step, transferring charges from the vertical charge transfer devices to the horizontal charge transfer device so that the remaining charge is added only to a charge obtained from the second area, wherein the discarding step, the reading step and the second charge transfer step are repeatedly performed during period when charges in horizontal lines, of the solid-state image sensing device, including charges obtained from the first area, are transferred from the vertical charge transfer devices to the horizontal charge transfer device.
According to the present invention, the foregoing object is also attained by providing a partial scanning method for reading a necessary charge from a predetermined first area of a solid-state image sensing device and discarding a unnecessary charge from a second area which is other than the first area, while scanning the solid-state image sensing device having pixel elements for storing charges, vertical charge transfer devices, arranged in parallel, for transferring charges stored in the pixel elements toward a horizontal charge transfer device, the horizontal charge transfer device for transferring charges to output the charges to outside of the solid-state image sensing device, and a buffer storage device provided between the vertical charge transfer devices and the horizontal charge transfer device, the method comprising: a first charge transfer step of transferring charges from the vertical charge transfer devices to the buffer storage device; a second charge transfer step of transferring charges from the buffer storage device to the horizontal charge transfer device; a discarding step of discarding charges, out of the charges transferred in the second charge transfer step, obtained from the second area by operating the horizontal charge transfer device; a reading step of reading charges, out of the charges transferred in the second charge transfer step, obtained from the first area by operating the horizontal charge transfer device; a third charge transfer step of transferring charges in the vertical charge transfer devices toward the buffer storage device in the discarding step; and a fourth charge transfer step of, when there is a remaining charge in the horizontal charge transfer device after the reading step, transferring charges from the buffer storage device to the horizontal charge transfer device so that the remaining charge is added only to a charge obtained from the second area, wherein the discarding step, the reading step, the third charge transfer step, and the fourth charge transfer step are repeatedly performed during period when charges in horizontal lines, of the solid-state image sensing device, including charges obtained from the first area, are transferred from the vertical charge transfer devices to the horizontal charge transfer device.
Further, the foregoing object is also attained by providing a partial scanning method for reading a necessary charge from a predetermined first area of a solid-state image sensing device and discarding a unnecessary charge from a second area which is other than the first area, while scanning the solid-state image sensing device having pixel elements for storing charges, vertical charge transfer devices, arranged in parallel, for transferring charges stored in the pixel elements toward a horizontal charge transfer device, and the horizontal charge transfer device for transferring charges to output the charges to outside of the solid-state image sensing device, the method comprising: a charge transfer step of, when there is a remaining charge in the vertical charge transfer devices after all the charges obtained from the first area are transferred from the vertical charge transfer device to the horizontal charge transfer device, transferring charges from the pixel elements to the vertical charge transfer devices so that the remaining charge is added only to a charge obtained from the second area.
Furthermore, the foregoing object is also attained by providing a solid-state image sensor comprising: pixel elements for storing charges; a horizontal charge transfer device for transferring charges to output the charges to outside of the solid-state image sensor; vertical charge transfer devices, arranged in parallel, for transferring charges stored in the pixel elements toward the horizontal charge transfer device; and a buffer storage device, provided between the vertical charge transfer devices and the horizontal charge transfer device, for enabling to operate the vertical charge transfer devices during operating the horizontal charge transfer device.
Further, the foregoing object is also attained by providing a solid-state image sensor comprising: pixel elements for storing charges; a horizontal charge transfer device for transferring charges to output the charges to outside of the solid-state image sensor; vertical charge transfer devices, arranged in parallel, for transferring charges stored in the pixel elements toward the horizontal charge transfer device; and a gate provided between the vertical charge transfer devices and the horizontal charge transfer device.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.